Efficient enzyme: bacteria that eat CO₂

Carbon dioxide (CO2) is the primary greenhouse gas on earth and contributes significantly to the steadily worsening climate crisis.

The development of efficient methods to remove CO2 from the atmosphere and reduce its emission is therefore extremely important.

For example, hydrogen (H2) is becoming more and more important as a potentially green energy source.

However, the use and storage of this gas is often still a challenge, because compared to other fuels, a significantly higher volume is required for the same energy gain.

A possible alternative is the production of H2 from formic acid, which can serve as a kind of “liquid hydrogen storage”.

Microbiologists from the Goethe University in Frankfurt, in cooperation with scientists from the universities in Basel and Marburg, have now succeeded in decoding the structure of a microbial enzyme from which biotechnology could learn a lot in the future: As a kind of "nanowire", it withdraws from the environment very efficiently CO2 and H2 and stores the gases in the form of formic acid in the cell, as the researchers report in the journal "Nature".

Unique bacterial enzyme

The extraordinary enzyme with the abbreviation

“HDCR” (“

-

dependent

CO2

reductase”) comes from the heat-loving bacterium

discovered in 2013 by microbiologists led by Volker Müller from the Goethe University in Frankfurt.

occurs in the deep sea - the bacteria therefore live far away from any oxygen.

The cells are adapted to these extreme conditions.

They use the existing gases CO2 and H2 as an energy source.

The reaction that takes place is catalyzed by HDCR: Electrons from the hydrogen are transferred directly to the carbon dioxide, resulting in the formation of formic acid.

Unlike all other known enzymes that can form formic acid, HDCR can use hydrogen directly and does not require an additional source of electrons.

In addition, at room temperature it is more than a thousand times more efficient than all other known catalysts, which often require high pressure of not infrequently 40 bar and high temperatures of 120 degrees.

Another important advantage is

Filaments enable highly efficient catalysis

In their experiments, the scientists discovered that under laboratory conditions, HDCR forms long threads, so-called filaments, that are decorated with enzymes.

In this case, modules that have the corresponding catalytic functions are repeated.

So there is a protein module that splits hydrogen, one that forms formic acid, and two small ones that contain iron and sulfur atoms.

The researchers recognized that the formation of the filaments plays a decisive role because it strongly stimulates the enzyme activity, says Müller.

But how exactly the filaments enable such high efficiency has so far been a mystery.

In close cooperation with the research group of Jan Schuller from Phillips University and the LOEWE Center for Synthetic Microbiology in Marburg, the scientists have now succeeded in taking a molecular close-up of the enzyme using cryo-electron microscopy.

The most precise details became visible for the first time: The backbone of the filaments consists of a series of small modules that form a kind of "nanowire" with thousands of electron-conducting iron and sulfur atoms.

In this way, the two catalytic modules required for the reaction are optimally connected to one another, like a motorway.

Efficient CO2 storage seems to be made possible by this special spatial structure.

look at.

What they saw there surprised the researchers.

Not only were they able to confirm the formation of HDCR filaments, they also found that hundreds of these filaments are twisted around each other like a braid and form large ring-shaped bundles that are anchored in the membrane.

Storage of electrons in filament bundles

However, the precise physiological function of this superstructure remains speculative and requires further investigation.

However, the scientists suspect that the bundles help to stabilize the filaments and could serve as a starting point for new filament formation.

"The hydrogen concentrations in the ecosystem of these bacteria are low, and the CO2 and H2 concentrations can also change," explains Müller.

The formation and, moreover, the bundling of the filaments not only created a significant increase in the concentration of these enzymes in the cell.

Thousands of electron-conducting iron atoms in this "nanowire" could also temporarily store the electrons from the hydrogen oxidation when a hydrogen bubble is just passing the bacteria, says Müller.

However, how the electrons are stored still needs to be investigated in the future.

The application of such biological systems in biotechnology could help in the fight against climate change in the future.